Probing the Intermediates of Halogen Addition to Alkynes: Bridged

Department of Chemistry, Kent State University, Kent, Ohio 44242, and Department ... solvation effects on the relative stabilities in the resulting br...
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Probing the Intermediates of Halogen Addition to Alkynes: Bridged Halonium versus Open Halovinyl Cation; A Theoretical Study Takao Okazaki†,‡ and Kenneth K. Laali*,† Department of Chemistry, Kent State University, Kent, Ohio 44242, and Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan [email protected] Received April 27, 2005

Intermediates formed in halogen addition (X ) Br, Cl, F) to alkynes (ethyne, propyne, 2-butyne, trifluoromethylethyne, trimethylsilylethyne, and 1-trimethylsilylpropyne) were studied computationally by MP2 at the MP2/6-311++G(3df,3pd) level and/or by DFT at the B3LYP/6-31+G(d) level. Structure optimization and frequency calculations were performed to identify the minima and to obtain their relative energies. PCM calculations (with H2O, CH2Cl2, and CCl4 as model solvents) were employed to examine solvation effects on the relative stabilities in the resulting bridged halonium, β-halovinyl, or R-halovinyl cations. GIAO-MP2 and GIAO-DFT calculations were employed to compute NMR chemical shifts (13C, 19F, and 29Si as appropriate). In selected cases, PCM-GIAO calculations were also performed to investigate the extent of solvent effects on the computed NMR shifts. The NPA-derived charges and the GIAO shifts were examined in comparative cases to shed light on structural features. In several cases, structure optimization starting with the β-halovinyl cations resulted in R-halovinyl cations (via formal hydride shift or trimethylsilyl shift). With the CF3 derivative (when X ) F), a formal F shift results in polyfluoroallyl cation generation from fluorovinyl cation as starting geometry.

Introduction In halogenation of alkenes, formation of β-halocarbocations a or ethylenehalonium ions b and their interplay (Figure 1a) is a well-investigated classical “organic textbook” problem.1 Apart from kinetics and stereochemical studies, a relatively large number of cyclic halonium * To whom correspondence should be addressed. Phone: 330-6722988. Fax: 330-672-3816. † Kent State University. ‡ Kyoto University. (1) (a) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, 4th ed.; Kluwer Academic/Plenum Publishers: New York, 2000; Part A, Chapter 6. (b) Carroll, F. A. Perspectives on Structure and Mechanism in Organic Chemistry; Brooks/Cole: Pacific Grove, CA, 1998; Chapter 9. (c) Isaacs, N. S. Physical Organic Chemistry, 2nd ed.; LongmanScientific & Technical: Essex, U.K., 1995; Chapter 12. (d) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper: New York, 1987; Chapter 7.

ions (type b) were generated by Olah and associates from suitable precursors under stable ion conditions and studied directly by NMR techniques; some were found to be in equilibrium with the open β-halocarbocations (type a), and others could be isolated as stable salts.2-4 The structural/mechanistic aspects of alkene bromination were discussed in a detailed review by Ruasse.5 Damrauer, Leavall, and Hadad6 reported a theoretical (2) Olah, G. A.; Laali, K. K.; Wang, Q.; Prakash, G. K. S. Onium Ions; Wiley: New York, 1998; Chapter 6. (3) Olah, G. A. Halonium Ions; Wiley: New York, 1975. (4) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; Wiley: New York, 1985; Chapter 4. (5) Ruasse, M.-F. In Advances in Physcial Organic Chemistry; Bethel, D., Ed.; Academic Press: New York, 1993; Vol. 28, pp 207291. (6) Damrauer, R.; Leavell, M. D.; Hadad, C. M. J. Org. Chem. 1998, 63, 9476.

10.1021/jo050848b CCC: $30.25 © 2005 American Chemical Society

Published on Web 10/11/2005

J. Org. Chem. 2005, 70, 9139-9146

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FIGURE 1. (a) Mechanism for reactions of alkene and alkyne with halogens. (b) Rearrangement of β-halovinyl cations to R-halovinyl cations.

study of cyclopentene and cyclohexene halonium ions. In comparison, a limited number of studies have dealt with halogenation of alkynes as a means to access β-halovinyl cation c and/or the unsaturated cyclic (bridged) halonium ion d.7 Kinetic studies and product analysis indicated vinyl cation-chloride ion pair formation in chlorination of terminal and disubstituted alkynes.7 The open vinyl cation is also the likely intermediate in bromination of phenylacetylenes.7 The AdE3 mechanism has been invoked in the presence of added salt (LiBr). On the other hand, predominant formation of the anti-dibromo adducts and absence of syn or solvent-incorporated products suggested that d should be a key intermediate.7 In bromination of ring-substituted 3-arylpropynes, depending on R, either a cyclic phenonium ion or a cyclic bromirenium (bridged halonium) ion is formed, resulting in predominant formation of the syn- and anti-dibromides, respectively.7 It has been stated that alkynes with alkyl substituents at the triple bond are more likely to go through a cyclic onium ion d, whereas phenylacetylenes would go through the open vinyl cation e, with the tendency for cyclization increasing in the order I > Br > Cl (corresponding to the bridging ability of halogens).1b,7 Chiappe et al.8a showed that bromination of alkynes with Br2 in [BMIM][Br] ionic liquid is anti-stereospecific irrespective of the nature of substituents at the triple bond, whereas the same reaction in [BMIM][PF6] gave syn/anti mixtures. On the basis of kinetic studies,8b involvement of an open vinyl cation (type c in Figure 1a) (7) (a) Stang, P. J.; Rappoport, Z.; Hanack, M.; Subramanian, L. R. Vinyl Cations; Academic Press: New York, 1979. (b) Lucchini, V.; Modena, G.; Pasquato, L. In Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.; Wiley: New York, 1997; Chapter 6. (8) (a) Chiappe, C.; Capraro, D.; Conte, V.; Pieraccini, D. Org. Lett. 2001, 3, 1061. (b) Chiappe, C.; Conte, V.; Pieraccini, D. Eur. J. Chem. 2002, 2831.

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was invoked in bromination of PhCtCR and a bridged bromirenium ion (type d, Figure 1a) in the case of p-CF3PhCtCR in [BMIM][PF6] solvent.8b Bromination of alkynylsilanes including HCtCTMS with Br2 in CCl4 under mild conditions was reported to produce the corresponding anti-dibromovinylsilanes.9 A number of long-lived vinyl cations, in particular the β-silyl-stabilized analogues, have been generated under stable ion conditions mainly by protonation of suitably substituted alkynes. Authoritative progress reviews on the topic by Siehl10a,b and by Siehl and Mu¨ller10c are available. To our knowledge, there are as yet no examples of bridged halonium ions of types d and f (Figure 1a) as NMR observable species. Similarly, for the open vinyl cations, despite an impressive literature on persistent vinyl cations,10 no examples seem to exist for β-halovinyl cations of type i or iii (Figure 1b) nor on R-halovinyl cations as in ii or iv (Figure 1b), and no extensive theoretical studies are available to serve as guide to their relative stabilities (β-halovinyl versus R-halovinyl) and possible interconversion as a function of halogen and cation structure. Parent vinyl cation (protonated acetylene) itself is predicted to be bridged (as in d with X ) H).11 A theoretical study on the C2H2F+ cation was reported,12 showing that the cyclic ion is a transition state and the open β-fluorovinyl cation is 31 kcal/mol lower in energy. The importance of polarization functions and electron correlation has been emphasized by Hamilton and Schaeffer for the C2H2Cl+ and C2H2Br+ cations,13 illustrating that at the more advanced levels the open halovinyl cations become shallow minima or transition states. Recent progress in high level theoretical studies of vinyl cations and their isomers, including C2H2X+ ions, has been discussed in a review by Apeloig and Mu¨ller,14a and more recent computational studies have been reported by van Alem, Lodder, and Zuilhof.14b,c In an effort to obtain a more comprehensive picture as to the relative stabilities of halogen cations c versus d, and i/iii versus ii/iv as a function of alkyne structure and the nature of halogen, we performed an extensive theoretical study by DFT and MP2 methods, focusing on the bridged halonium ions and the open vinyl cations formed when halogens (X ) Br, Cl, F) react with ethyne, propyne, 2-butyne, trifluoromethylethyne, trimethylsilylethyne and 1-trimethylsilylpropyne. The influence of solvation (in CH2Cl2, CCl4, and H2O) on the relative (9) Al-Hassan, M. I. J. Organomet. Chem. 1989, 372, 183. (10) (a) Siehl, H.-U. In Stable Carbocation Chemistry; Prakash, G. K. S., Schleyer, P. v. R., Eds.; Wiley: New York, 1997; Chapter 5. (b) Siehl, H.-U. In Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.; Wiley: New York, 1997; Chapter 5. (c) Siehl, H.-U.; Mu¨ller, T. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998; Vol. 2, Part 1, Chapter 12. (11) (a) Weber, J.; Yoshimine, M.; McLean, A. D. J. Chem. Phys. 1976, 64, 4159. (b) Lindh, R.; Rice, J. E.; Lee, T. J. J. Chem. Phys. 1991, 94, 8008. (12) Csizmadia, I. G.; Luchini, U.; Modena, G. Theor. Chim. Acta 1975, 39, 51. (13) Hamilton, T. P.; Schaefer, H.-F., III. J. Am. Chem. Soc. 1991, 113, 7147. (14) (a) Apeloig, Y.; Mu¨ller, T. In Dicordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.; Wiley: New York, 1997; Chapter 2. (b) van Alem, K.; Lodder, G.; Zuilhof, H. J. Phys. Chem. A 2000, 104, 2780. (c) van Alem, K.; Lodder, G.; Zuilhof, H. J. Phys. Chem. A 2002, 106, 10681.

Bridged Halonium versus Open Halovinyl Cation

stabilities of the two types of intermediates was probed in selected cases via the PCM method. The GIAO-derived NMR chemical shifts and the NPA-derived charges were computed for the resulting minima. In selected cases, solvent effects were also examined on the NMR chemical shifts and on charge distribution. It was hoped that the collective data would help with the design of experiments that could lead to direct observation and NMR study of the cyclic cations of type d and/or halovinyl cations of types i-iv. Results and Discussion Computational Protocols. Structures were optimized using molecular point groups shown in Tables 1 and S2-7 (Supporting Information) by the density functional theory (DFT) method at B3LYP/6-31+G(d) level and the Møller-Plesset perturbation theory at MP2/6311++G(3df,3pd) level using the Gaussian 03 package.15 Solvent effects were included using the polarizable continuum model (PCM),16 and the dielectric constants of 78.39, 8.93, and 2.228 were used for H2O, CH2Cl2, and CCl4. Computed geometries were verified by frequency calculations. Furthermore, global minima were checked by manually changing initial geometries and by comparing the resulting optimized structures and their energies. NMR chemical shifts were calculated by the gaugeincluding atomic orbital (GIAO)17,18 method at the B3LYP/ 6-31+G(d)//B3LYP/6-31+G(d) or the MP2/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level. NMR chemical shifts were referenced to CH3F for δ13C (71.6 ppm) and δ19F (-271.9 ppm) and to TMS for δ29Si (0 ppm) calculated with a molecular symmetry of C3v or Td at the same level of theory (Table S1). At the onset, GIAO-derived isotropic magnetic shielding tensors were calculated for CH4, MeF, Me4Si, CCl3F, C6H6, and C6F6 by B3LYP/631+G(d) and for CH4 and MeF at the MP2/6-311++G(3df,3pd) level. As Table S3 illustrates, the computed values obtained for these reference compounds were in good overall agreement with their reported experimental values, except for CCl3F, whose GIAO-derived δ13C deviated noticeably from the reported experimental value. On the basis of these “inter-referencing” tests (Table S1), MeF was selected for calibration and refer(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (16) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027. (17) (a) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251. (b) Dichfield, R. Mol. Phys. 1974, 27, 789. (18) Wolfram, K.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2000; Chapter 11.

encing of the GIAO-derived 13C and 19F NMR chemical shifts at both MP2 and DFT levels (see earlier discussion). As a test of basis-set dependency of GIAO chemical shifts,17 the bridged cations 1Br and 1Cl, the β-halovinyl cation 2F, and the R-halovinyl cations 3Br, 3Cl, and 3F (see later discussion) were computed at various DFT and MP2 levels. The data are summarized in Figure S1a. It can be seen that the largest changes are for the C+ centers in the vinyl cations, with the MP2-derived shifts being less deshielded than those from DFT. For the bridged halonium ions, the variations are less significant, with the MP2-derived values being smaller. These tests indicated that the employed levels were adequate, especially for comparative purposes to derive trends. Tables 1 and S2-7 (in Supporting Information) summarize the total electronic energies (E), zero point energies (ZPE), and relative energies (∆E) for the studied compounds. Natural population analysis (NPA)-derived charges were computed at the same levels. Reactive Intermediates of Halogenation of Ethyne; the C2H2X+ Cations. Although relative energies and geometries for the C2H2X+ ions had previously been computed at the correlated levels and were discussed in ref 13, solvent effects and computed NMR chemical shifts were not available. Siehl and associates19 examined the influence of basis set on computed NMR chemical shifts for several R-vinyl-substituted vinyl cations (dienyl cations). The closest values to experimental NMR values were obtained at correlated levels [MP2 and CCSD(T)], whereas HF-SCF calculations were unsatisfactory. More recently, Mu¨ller and co-workers20 reported a reasonable linear correlation between experimental and computed 13C NMR chemical shifts by GIAO-DFT in a series of R-aryl-β-disilyl-substituted vinyl cations. Our study began with a reexamination of C2H2X+ ions (i.e., the reactive intermediates of halogen addition to parent acetylene). Results of E, ZPE, and ∆E for the derived cyclic halonium ions and the vinyl cations are summarized in Table 1 along with their molecular symmetry. The cyclic bromonium 1Br and cyclic chloronium 1Cl ions are minima (C2v symmetry) at both B3LYP/6-31+G(d) and MP2/6-311++G(3df,3pd) levels (Figure 2). The cyclic fluoronium ion 1F is minimum at the MP2/6-311++G(3df,3pd) level and a transition state (TS) at the B3LYP/ 6-31+G(d) level (1F was found to be a TS at the MP3/ 6-31G*//6-31G* level).14a Focusing on the open vinyl cations, we found that the β-bromo 2Br and the β-chlorovinyl cations 2Cl did not exist as stable species (see later discussion), but the β-fluorovinyl cation 2F is minimum. Geometry optimization on 2Br and 2Cl as starting geometries resulted in rearrangement (a formal hydride shift) to produce the corresponding R-halovinyl cations 3Br and 3Cl, which are minima at both B3LYP/6-31+G(d) and MP2/6311++G(3df,3pd) levels. (19) (a) Siehl, H.-U.; Mu¨ller, T.; Gauss, J. J. Phys. Org. Chem. 2003, 16, 577. (b) Siehl, H.-U.; Brixner, S. J. Phys. Org. Chem. 2004, 17, 1039. (20) Mu¨ller, T.; Margraf, D.; Syha, Y. J. Am. Chem. Soc. 2005, 127, 10852.

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Okazaki and Laali TABLE 1. Electronic Energies (E), Zero Point Energies (ZPE), and Relative Energies (∆E) for 1Br, 3Br, 1Cl, 2Cl, 3Cl, 1F, 2F, and 3F by MP2/6-311++G(3df,3pd) and/or by B3LYP/6-31+G(d)

cation

method

1Br

B3LYP/6-31+G(d) MP2/6-311++G(3df,3pd) B3LYP/6-31+G(d) MP2/6-311++G(3df,3pd) B3LYP/6-31+G(d) MP2/6-311++G(3df,3pd) B3LYP/6-31+G(d) B3LYP/6-31+G(d)

3Br 1Cl 2Cl 3Cl

1F 2F 3F a

molecular point group

E, hartree

ZPE hartree

no. of imaginary frequencies

∆E,a kcal/mol

C2v C2v Cs C2v C2v C2v Cs Cs C2v C2v C2v C2v Cs C2v C2v

-2648.7079359 -2649.3565994 -2648.7426163 -2649.3712008 -537.1667424 -536.5005387 -537.1586864 -537.2050724 -537.2050724 -536.5299165 -176.7550981 -176.4569861 -176.7793393 -176.8177664 -176.5200878

0.028262 0.028422 0.028713 0.028006 0.028519 0.028913 0.027511 0.028714 0.028715 0.028727 0.028703 0.028844 0.028758 0.029891 0.029783

0 0 0 0 0 0 1 0 0 0 1 0 0 0 0

(0) (0) -21.8 -9.2 (0) (0) 5.1 -24.1 -24.1 -18.4 (0) (0) -15.2 -39.3 -39.6

MP2/6-311++G(3df,3pd) B3LYP/6-31+G(d) MP2/6-311++G(3df,3pd) B3LYP/6-31+G(d) B3LYP/6-31+G(d) MP2/6-311++G(3df,3pd)

Relative energy (∆E) compared to bridged cations, 1Br, 1Cl, and 1F, at the same level.

FIGURE 2. Intermediates of halogen addition to acetylene.

The rearranged open vinyl cation 3Br is strongly favored relative to the cyclic ion 1Br (by 21.8 kcal/mol at the B3LYP/6-31+G(d) level and by 9.2 kcal/mol at the MP2/6-311++G(3df,3pd) level; an energy difference of 13.8 kcal/mol was computed at the CISD(+Q)/DZ+d// CISD/DZ+d level).14a Similarly, the rearranged open vinyl cation 3Cl is more stable than the bridged ion 1Cl (by 24.1 kcal/mol at the B3LYP/6-31+G(d) level and by 18.4 kcal/mol at the MP2/6-311++G(3df,3pd) level; the difference was computed to be 20.6 kcal/mol by MP4/6311G(2df,p)//MP2/6-311G**+ZPE+ thermal energy).14a Whereas the β-fluorovinyl cation 2F and the R-fluorovinyl cation 3F are both minima at the B3LYP/6-31+G(d) level, 3F is noticeably more stable. Overall, in agreement with previous findings included in ref 14a, from an energetic standpoint, the R-halovinyl cations are the preferred intermediates in halogen addition to acetylene, but the relative importance of bridged halonium ion 1Br and to a lesser extent 1Cl should not be overlooked. The GIAO-derived 13C NMR chemical shifts and the NPA-derived charges for the cyclic halonium ions 1Br, 1Cl, and 1F, and for the open vinyl cations 3Br, 3Cl, 2F, and 3F were computed at the MP2/6-311++G(3df,3pd) and/or B3LYP/6-31+G(d) level. These data along with the optimized structures are summarized in Figure S1. The NPA-derived charges indicate that positive charge in 1Br and 1Cl resides on the halogens. In the fluoronium ion 1F, positive charge resides on the carbons, and fluorine bears negative charge. This trend is reflected in the computed 13C GIAO chemical shifts, which show a deshielding trend in the order 1F > 1Cl > 1Br. Computed δ 13C for the bridged bromonium ion 1Br is 90.5 ppm (by B3LYP) and 86.0 ppm (by MP2). The δ 13C for 9142 J. Org. Chem., Vol. 70, No. 23, 2005

the bridged chloronium ion 1Cl is 101.9 ppm (by B3LYP) and 96.6 ppm (by MP2), and the GIAO-derived δ 13C for the bridged fluoronium ion 1F is 174.2 ppm (by MP2). The bridged cations 1Br, 1Cl, and 1F are symmetrical with nearly identical C-C bond lengths and with the C-X bonds changing from 2.01 Å (C-Br) to 1.86 (C-Cl) to 1.56 Å (C-F) (by MP2; see Figure S1). Cyclic halonium ions are regarded as n-electron bridged species. NBO analysis indicated no 3-center bonding in 1Br, 1Cl, 2Cl (this is a TS), and 1F (also a TS) at the B3LYP/6-31+G(d) level. On the contrary, a 3-center bond is implied in 2F for H(1)-C(1)-C(2). The H(1) atom is bonded to the cationic carbon at the optimized geometry, and the H(1)-C(1)-C(2) angle is 104.4°. Focusing on the GIAO NMR data (in Figure S1), we see that the β-fluorovinyl cation 2F has an unusually deshielded C+ (at 453 ppm) and a shielded δ19F (at -260 ppm) by B3LYP as compared to R-fluorovinyl cation 3F whose C+ is at 242 ppm and δ19F at 81 ppm by MP2. This trend presumably reflects significant fluoronium ion character in 3F by p-π back-bonding. The computed 13C NMR chemical shift for the C+ in 3F (242.8 ppm) is in close range of the experimental value for Mes-C+dCH2 reported at 238.5 ppm.10b GIAO-derived δ13C values for 3Br are at 311.0 (Br+ C ) and 65.9 ppm (CH2) by B3LYP, and at 287.9 (BrC+) and 66.7 ppm (CH2) by MP2. For 3Cl these are at 296.7 (Cl-C+) and 54.8 ppm (CH2) by B3LYP, and at 274.2 (Cl-C+) and 54.6 (CH2) by MP2. Comparing these data with those of 3F reveals a shielding trend on both the vinylic carbocation center and the CH2 in the order Br < Cl < F, which correlates with the back-bonding ability of halogens in R-halocarbenium ions.21 But at the same time, consideration of the energetics of isodesmic reaction below (MP3 or CISD calculations)14 leads one to conclude that destabilization by σ-withdrawal versus stabilization by p-π back-bonding is largest for X ) F and smallest for X ) Br.

CH2dCH-X + CH2dC+-H f CH2dC+-X + C2H4 (for X ) F, ∆E ) 6.1; for X ) Cl, ∆E ) -8.4; for X ) Br, ∆E ) -14.1 kcal/mol)

Bridged Halonium versus Open Halovinyl Cation

Solvent Effect on Relative Stabilities; PCM Calculations for C2H2X+ Cations. To gather some insight into solvation effects on relative stabilities of the open versus cyclic cations, the cyclic halonium ions 1Br, 1Cl, 1F and the halovinyl cations 3Br, 3Cl, 2F, and 3F were re-optimized in H2O, CH2Cl2, and CCl4 as solvent using the PCM method. The data are summarized in Table S2. It can be seen that solvent has a significant energy lowering effect on the cations in the order H2O > CH2Cl2 > CCl4. Among the bridged halonium ions, the stabilization order 1F > 1Cl > 1Br is observed. Geometry optimization on 2F (β-fluorovinyl cation) in water afforded 3F (R-fluorovinyl cation) as the final geometry. As Table S2 illustrates, vinyl cations are also strongly stabilized by solvation, but to a lesser extent as compared to their cyclic counterparts. These model PCM calculations imply that bridged halonium ions C2H2X+ enjoy significant stabilization in solution, and this underscores their relevance in solution studies. There are no notable changes in the optimized structures and in the NPA charges in the bridged cations or the vinyl cations when solvation effects are implemented (Figure S2). There are only small changes in the GIAOδ13C values, and the resulting changes are in the order H2O > CH2Cl2 > CCl4, which corresponds to the dielectric constants. Whereas the GIAO 13C chemical shifts for 1Br and 1Cl are only slightly affected by solvent, magnitude of positive charge at halogen is diminished in the order H2O > CH2Cl2 > CCl4, according with the polarities. Whereas solvent effects on 13C chemical shift changes in the R-halovinyl cations 3Br, 3Cl, and 3F are relatively small, large changes were computed for the β-fluorovinyl cation 2F between CH2Cl2 and CCl4 (geometry optimization on 2F in H2O afforded 3F). Reactive Intermediates of Halogenation of Propyne; the C2H(Me)X+ Cations. Upon structure optimizations at the B3LYP/6-31+G(d) level, bridged halonium ions derived from propyne (4Br, 4Cl, 4F and 4aBr, 4aCl, 4aF) converged to open vinyl cations (5Br, 5Cl, 5F and 5aBr, 5aCl, 5aF) (Figure 3). The energy data for the gas phase as well as those calculated by the PCM method in H2O, CH2Cl2, and CCl4 as representative solvents are gathered in Table S3 for comparison. For 5Br, PCM calculations in H2O and CH2Cl2 gave structures with one imaginary frequency, whereas all other structures were minima in all solvents. PCM calculations indicate a stabilization trend for this group similar to those found for 3Br, 3Cl, and 3F (i.e., H2O > CH2Cl2 > CCl4) (Table S3). For comparison, the corresponding R-halovinyl cations, 6Br, 6Cl, and 6F were also calculated and found to be minima (Table S3). Whereas 6Br and 6Cl are more stable than 5Br and 5Cl (by 8.3 and 4.7 kcal/mol, respectively), 6F is less stable than 5aF (by 1.9 kcal/mol). Species 6aBr, 6aCl, and 6aF were found to be transition states to methyl rotation in 6Br, 6Cl, and 6F. On the basis of these findings, the β-halovinyl and the R-halovinyl cations are both minima in halogenations of propyne (these vinyl cations are readily interconvertible via 1,2-H shifts), and no bridged cations were obtained as final structures at this level. (21) Olah, G. A.; Mo, Y. K.; Halpern, Y. J. Am. Chem. Soc. 1972, 94, 3551.

FIGURE 3. Halonium ions and vinyl cations by halogen addition to propyne.

GIAO-derived NMR chemical shifts, NPA-derived charges, and geometries for the β-halovinyl cations derived from propyne are summarized in Figure S3, whereas data for the R-halovinyl cations are summarized in Figure S3a. For the β-halovinyl cations (5Br, 5Cl, and 5aF), the GIAO-δ13C values at the vinylic carbocation (and the CHX) follow the order Br < Cl < F, but the reverse order is observed in the R-halovinyl cations (6Br, 6Cl, and 6F). The NPA charge at the vinylic carbocation in the β-halovinyl cations are computed to be 0.404 (for 5Br), 0.478 (for 5aBr), 0.502 (for 5Cl), 0.506 (for 5aCl), 0.458 (for 5F), and 0.442 (for 5aF), and these values decrease in the order H2O > CH2Cl2 > CCl4. For the R-halovinyl cations, the NPA-derived charges at the vinylic carbocations are 0.108 (in 6Br), 0.228 (in 6Cl), and 0.885 (in 6F). The dihedral angles for H-C(3)-C(1)-X are 180° in 5Br, 5Cl, and 5F and 0° in 5aBr, 5aCl, and 5aF, and based on relative energies the former group (with the exception of 5F) is slightly more stable. In the optimized structures, the vinylic moiety (C/C double bond) becomes longer and the Me-C bond becomes shorter in the sequence F > Cl > Br (Figure S3). NBO analysis for 5Cl and 5aF implies a 3-center bonding in C(2)-C(3)-H(3), with a bond angle of less than 120°. Reactive Intermediates of Halogenation of 2-Butyne; the (Me)2C2X+ Cations. The bridged halonium ions and the open vinyl cations shown in Figure 4 were computed by DFT, and energies of their optimized structures are summarized in Table S4. Among the bridged cations (Figure 4), 7Br, 7Cl, and 7F had the lowest energies, but frequency analysis indicated that they were transition states (one imaginary frequency) to the formation of the β-halovinyl cations 8Br, 8Cl, 8cCl, and 8cF, which were minima (had no imaginary freJ. Org. Chem, Vol. 70, No. 23, 2005 9143

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FIGURE 4. Halonium ions and vinyl cations as intermediates in halogen addition to 2-butyne.

quency). Geometry optimization on 7bBr (included in Figure 4) as initial structure resulted in 8cBr as the final structure. The R-halovinyl cations 9Br, 9Cl, and 9F are formed by methyl migration via 8Br, 8Cl (or 8cCl), and 8cF. Whereas energies of 9Br versus 8Br and 9Cl versus 8Cl are rather close (marginally favoring the R-halovinyl cation), in the case of fluorinated vinyl cations the β-fluoro analogue 8cF is strongly preferred relative to R-fluorovinyl cation 9F (by 8.3 kcal/mol). Their GIAO-derived NMR chemical shifts, NPAderived charges, and the optimized structures with bond lengths are summarized in Figure S4. Computed GIAO-δ13C values for the vinylic carbocation (and the CHX carbon) at the cationic centers increase in the order F > Cl > Br in the β-halovinyl cations (8Br, 8Cl, 8cCl, and 8cF), while the opposite trend is seen in the R-halovinyl cations (9Br, 9Cl, and 9F) (Figure S4). Comparison of the GIAO shifts in R-halovinyl cations 3X, 6X, and 9X (X ) Br, Cl, and F) indicates fairly regular methyl substituent effects. But methyl substitution in the β-halovinyl cations 5X and 8X induces dramatic shielding effect on the vinylic cation centers, in the case of X ) Br and X ) Cl, but little effect is seen when X ) F. Inspection of the X-CdC angles (100.3° for 5Br, 119.1° for 5Cl, 124.1° for 5aF, 89.3° for 8Br, 103.1° for 8Cl, and 120.9° for 8cF, and increasing in the order F > Cl > Br) provides some insight, pointing to partial bridging by bromine and chlorine in the more crowded analogues as an important contributing factor to vinyl cation shielding. It is also noteworthy that in the optimized structures the C-X bond (X ) Br, Cl) is elongated in 8X relative to 5X. Among the β-fluorovinyl cations, the 19F chemical shift in 2F and 5aF are nearly the same but that in 8cF is ∼55 ppm more downfield. Finally, the computed NPA charge at the vinylic cation center is 0.304 (for 8Br), 0.437 (for 8Cl), and 0.431 (for 8cF), which, in comparison with 5Br, 5Cl, and 5aF, has diminished by 0.100 (for 8Br), 0.065 (for 8Cl), and 0.011 for (8cF). 9144 J. Org. Chem., Vol. 70, No. 23, 2005

FIGURE 5. Halonium ions and vinyl cations intermediates in halogen addition to trifluoromethylacetylene.

Intermediates of Halogenation of Trifluoromethylethyne; C2H(CF3)X+ Cations. The bridged halonium ions (10Br-10F and 10aBr-10aF) and the open vinyl cations 11Br-11F and 11aBr-11aF were calculated by DFT as initial structures (Figure 5), and energies of their optimized structures are summarized in Table S5. In the case of Br-substituted cations, only the bridged species 10Br, 10aBr, and the rearranged vinyl cation 12Br were obtained as final structures, and structures 11Br or 11aBr could not be obtained at the B3LYP/631+G(d) level. Frequency analyses showed that 10Br and 12Br are minima, but 10aBr is a transition state for rotation of a CF3 group in 10Br. For the Cl-substituted cations, 10Cl, 10aCl, 11aCl, and 12Cl were obtained as optimized geometries, and frequency analyses indicated that only 10Cl, 11aCl, and 12Cl are minima. It is noteworthy that in this case the bridged ion 10Cl is more stable than the open vinyl cation 11aCl. A notable feature for the bridged cations 10Br and 10Cl is that they are unsymmetrical, with the CF3C-X bond being shorter. For X ) F, neither the bridged cation nor the open “unrearranged” vinyl cation could be obtained as optimized structures. Structure optimization resulted in rearranged species 12F, 13F, 14aF-14dF as final structures. Among these, only 12F, 14cF, and 14dF (isomeric polyfluorinated allyl cations) are minima. A 1,2-fluorine shift as found for 11F has already been examined in CF3C+dCHR (R ) H or Ph) computationally and experimentally.22 Their GIAO-derived NMR chemical shifts, NPAderived charges, and optimized structures including bond lengths are gathered in Figure S5. The allylic nature of the rearranged cations 14cF and 14dF is clearly evident from these data, in particular deshielding at both C1/C3 and deshielding of their fluorines (GIAO NMR). NPA (22) (a) McAllister, M.; Tidwell, T. T.; Peterson, M. R.; Csizmadia, I. G. J. Org. Chem. 1991, 56, 575. (b) van Alem, K.; Belder, G.; Lodder, G.; Zuilhof, H. J. Org. Chem. 2005, 70, 179.

Bridged Halonium versus Open Halovinyl Cation

FIGURE 7. Cyclic bromonium ions and vinyl cations interFIGURE 6. Halonium ions and vinyl cations intermediates

mediates in bromine addition to 1-trimethylsilylpropyne.

in halogen addition to trimethylsilylacetylene.

charges indicate that the allylic carbocation bearing two fluorines carries significantly more positive charge relative to that having one fluorine. Comparison between the computed GIAO NMR shifts in 1Br and 10Br shows that the halonium carbon attached to CF3 is more shielded relative to the unsubstituted halonium carbon, but such an effect is not implied in comparing 1Cl with 10Cl where the CF3 substituion appears to have little or no effect. Intermediates of Bromination of Trimethylsilylethyne; TMSC2HX+. The bridged halonium ions (15a and 15b) and the open vinyl cations (16a-d) were calculated by DFT as initial structures (Figure 6). The bridged ion 15a, the R-bromo-β-silylvinyl cation 17a, and the rearranged cations 17b and 17c were obtained as final structures at the B3LYP/6-31+G(d) level. Frequency analyses showed that 15a and 17a-c are minima (energies of the optimized structures are summarized in Table S6). It is seen that the rearranged cations 17b and 17c and the vinyl cation 17a are strongly preferred over 15a. Their GIAO-derived NMR chemical shifts, NPA-derived charges, and optimized structures for 15a and 17a-c including bond lengths are gathered in Figure S6. The optimized structure of 15a shows a distorted bridged cation (as in 10Br but more distorted), with the C-Br bond attached to the -SiMe3 group much elongated. GIAO 13C NMR implies that the TMS group has a notable deshielding effect on the halonium ion. Taken together, the optimized structure, NPA charges, and the GIAO 29Si and 13C shifts in 17a (compare for example the NMR of 17a with 6Br) point to silicon participation

and siliconium ion character in 17a. Cation 17b has the characteristics of a sila-allyl cation, whereas 17c could be viewed as a silicenium species with bromonium ion character (bromine participation). Intermediates of Bromination of 1-Trimethylsilylpropyne; TMSC2(Me)X+. Structures of 18a-d, 19a-d, and 20b (Figure 7) with restriction of Cs symmetry and 20a with C1 symmetry were optimized, and their energies are summarized in Table S7. The bridged bromonium ion 18a, the β-silyl-substituted β-bromovinyl cations 19a, and β-silyl-substituted R-bromovinyl cation 20a were found to be minima by frequency calculations with a relative stability order 20a > 19a > 18a. Structure 19b was computed to be a TS for methyl rotation in 19a, and optimization of structures 19c and 19d as initial structures resulted in 20a and 20b during optimization (TMS group migration). GIAO-derived NMR chemical shifts, NPA-derived charges, and optimized structures including bond lengths for 18a, 19a, and 20a are gathered in Figure S7. GIAO 29Si shift in 19a (81.4 ppm) is noticeably more deshielded relative to the bridged analogue 18a (16.4 ppm), and the GIAO 13C for C+ in 19a is shielded relative to that in 8Br (144 versus 188 ppm). At the same time, the NPA charge at C+ in 19a has diminished relative to that in 8Br (0.211 versus 0.304). These features are consistent with silicon bridging and imply considerable siliconium ion character in 19a. Comparison of the optimized geometries for 18a and 19a is also instructive, especially significant lengthening of the C-Si bond in 19a. Formally, cation 20a results from 19a by a trimethylsilyl group shift. Relative orientation of the C-Si J. Org. Chem, Vol. 70, No. 23, 2005 9145

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bond in 19a and 20a in the optimized structures seems well-suited to silicon hyperconjugation with the vinyl cation. Comparing the geometries and the GIAO NMR data in 20a and 17a is also instructive. Increased positive charge at silicon in 20a (NPA), longer C-Si and C-Br bonds and shorter CdC bond, and a more shielded vinyl cation (GIAO 13C) in 20a relative to that of 17a imply a higher degree of silicon bridging and siliconium ion character in the more crowed vinyl cation. This feature has previously been deduced experimentally based on the NMR data in silyl-substituted dienyl cations.10b Comparative Discussion and Summary Focusing first on halogenation of parent acetylene, the following key points emerge: For X ) Br, both R-bromovinyl cation 3Br and the bridged bromonium ion 1Br are minima, with the former being favored. A β-bromovinyl cation (2Br) is neither a TS nor an intermediate. For X ) Cl, a similar situation holds, but the energy gap between the bridged cation 1Cl and the β-chlorovinyl cation 3Cl is now much wider. For X ) F, whereas the bridged cation 1F is still a minimum, the difference in energy between 1F and the R-vinyl cation 3F is so large that involvement of 1F as an intermediate is highly unlikely. A notable difference with X ) F is that 2F (the β-fluorovinyl cation) is a minimum, but it is less stable relative to 3F. It can be concluded that the energy gap between the bridged and the open cations is smallest for X ) Br and largest for X ) F, and this conclusion is consistent with those summarized in ref 14a (based on CISD, MP2, MP3, and MP4 calculations). Involvement of bridged halonium ions (1Br, 1Cl, and even 1F) gains further credibility via PCM calculations showing that solvation has a strong energy lowering effect on these species, and the effect is typically in the order 1F > 1Cl > 1Br. The open vinyl cations are also stabilized by solvation effects but to a lesser extent, and the effects do not vary significantly as a function of X. In halogen addition to propyne, a different picture emerges, whereby the bridged halonium ions (4X or 4aX; X ) Br, Cl, F) are no longer important (neither as minima nor as TS) and β-halovinyl cations (5X and or 5aX) as well as R-halovinyl cations (6X) emerge as key intermediates. In halogen addition to symmetrical 2-butyne, the β-halovinyl cations (8X and/or 8aX) appear as most likely intermediates, followed closely by the corresponding R-halovinyl cations (9X; with X ) Br and Cl), whereas the bridged analogues (7X or 7aX) appear unimportant. Replacing the methyl group in propyne for -CF3 brings about interesting outcomes depending on X. With X )

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Br, the bridged cation 10Br and the open R-bromovinyl cation 12Br are both minima, but 12Br is significantly more stable. With X ) Cl, again both intermediates 10Cl and 11aCl are minima with the bridged structure 10Cl being slightly more stable. But with X ) F, neither the bridged nor the open “unrearranged” vinyl cations are minima, and skeletal rearrangement resulted in polyfluorinated allyl cations. A comparative study of bromine addition to trimethylsilylethyne indicated that whereas the bridged halonium ion 15a is minimum, the most likely intermediates are skeletally rearranged species (formed via formal -TMS or -Me migration) namely R-bromo-β-silylvinyl cations 17a, 17b, and 17c. Replacing a methyl in 2-butyne for a Me3Si group brings about interesting prospects in bromine addition. Both cyclic halonium 18a and the open β-bromo-βsilylvinyl cation 19a are minima, and the latter is more stable. A slightly more stable R-bromovinyl cation (20a) results via a formal TMS shift. The geometries, NPA charges, and GIAO NMR (13C, 19 F, 29Si) data have provided insight into the structures, charge distributions, and NMR chemical shifts for the structures that were found to be minima. In general, variations in the GIAO shifts were not significantly altered when solvation effects were implemented. The combined relative stability data and the NMR information should be helpful in designing experiments from appropriate alkynes under controlled/stable ion conditions in a search for persistent species of general type c and/or d (Figure 1a) and with structures i-iv (Figure 1b).

Acknowledgment. We are grateful to the reviewers for their careful review and for constructive comments and suggestions. This work was supported in part by the NCI of NIH (NCI 2 R15 CA07835-02A1) and by a Grant-in-Aid for Young Scientists (B) 15710160 from the Japan Society for the Promotion of Science (JSPS). Supporting Information Available: Table S1: GIAOderived isotropic magnetic shielding tensor and NMR chemical shifts for CH4, CH3F, TMS, CFCl3, C6H6, and C6F6. Tables S27: Electronic energies (E), zero point energies (ZPE), number of imaginary frequencies, and relative electronic energies (∆E) obtained from DFT (or DFT/PCM) calculations. Tables S884: Cartesian coordinates of the optimized structures by the DFT and MP2 methods. Figures S1-7: Computed NMR chemical shifts and NPA-derived charges and optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. JO050848B